The present invention relates to an emergency device for relighting an airplane turbojet that is windmilling, and more particularly when the jet has suffered an in-flight shutdown.
In the past, long-distance airliners have generally been fitted with four engines. Those airplanes have subsequently been replaced by airplanes propelled by only three engines. This trend towards reducing the number of engines on that type of airplane, together with the trend towards increasing the capacity of airplanes so as to reduce operating costs, has led to large capacity twinjet airplanes known as xe2x80x9clarge twinsxe2x80x9d being designed.
The engines fitted to that type of airplane have been developed so as to provide very high thrust with large bypass ratios. The increase in the compression ratio and the inertia of the high-pressure compressor in such an engine has also led to the speed at which the compressor rotates when windmilling decreasing for given flight conditions, and this has had the consequence of increasing the difficulty of relighting the combustion chamber in the event of a shutdown.
Furthermore, the use of twin jets for long flights over oceans increases the seriousness of the consequences in the event of an engine shutting down in flight. Furthermore, for various reasons, a twin jet can also be the subject of multiple shutdowns, something which happens extremely rarely, but is still possible. In the event of an in-flight shutdown, whether voluntary or involuntary, the engine can conserve the ability to restart. In the event of a single shutdown on a multi-jet airplane, the engine can be relighted by using the starter, relying on power delivered by the other engine(s) or by the auxiliary power unit. However, in the event of multiple extinctions for a common mode cause, the starter has no external power if the auxiliary power unit has stopped or been shut down, so it cannot be used, and the engine must be relighted while it is windmilling. The auxiliary power unit is a device that is conventionally used for powering the turbojet starter with electricity or compressed air.
The power available from the batteries of twin jets is limited, so emergency electrical systems cannot restart the engine. The power generated by the emergency air turbine is low (about 5 kilowatts (kW)) and the mass penalty associated with such a turbine is very large (about 100 kilograms (kg)), and it requires regular inspections.
The widespread use of long duration twin-engined flights over oceans, associated with the foreseeable development of engines towards ever-increasing bypass and compression ratios has thus had the consequence of increasing interest in devices that make it easier to relight engines in flight while they are windmilling.
For a turbojet having a large bypass ratio, when a shutdown occurs in flight, the turbines are no longer fed with heat energy, and the speed of rotation of the compressor and fan rotors decreases considerably in corresponding manner. Nevertheless, the relative wind (air flow) passing through the fan stage suffices to keep it rotating, which phenomenon is generally referred to by the term xe2x80x9cwindmillingxe2x80x9d. The high-pressure compressor receives only a small fraction of this relative wind, and therefore does not always rotate at a speed that is fast enough to allow the engine to be relighted without external assistance. The high-pressure compressor, when rotated by relative wind, needs to rotate quite quickly (N2 speed close to 20%) in order to deliver pressure greater than the minimum pressure for ignition, prior to admitting fuel and igniting it.
The fan is an advantageous source of energy, comprising both kinetic energy due to its own inertia and also wind energy extracted from the flow of air passing through it. Transferring a fraction of this energy to the high-pressure body then suffices to bring the high-pressure compressor up to a speed that allows the engine to be relighted. Thus, at all airplane speeds, power is available that can be transmitted from the low-pressure rotor (fan) to the high-pressure rotor (compressor).
It should be observed that the inertia and the energy from windmill operation of the low-pressure stage as a whole (fan and low-pressure turbine) are always much greater than those of the high-pressure stage (compressor and high-pressure turbine), and that future increases in bypass ratios will further increase this ratio.
Thus, U.S. Pat. No. 5,845,483 and French patent Nos. 2 315 259 and 2 351 266 disclose hydraulic, electrical, or pneumatic power generators driven by the fan for the purpose of transmitting the energy accumulated by the fan to the high-pressure compressor via a suitable device. Nevertheless, each of those relighting systems presents the major drawback of constituting an assembly that is complicated and bulky.
U.S. Pat. No. 5,349,814 discloses a gear train fitted with ratchet clutches or cam clutches connecting the shaft of the lower-pressure turbine to the shaft of the high-pressure turbine to transmit torque from the fan at predetermined engagement and disengagement speeds. However, that relighting device is not very reliable. It is very difficult to calibrate, so there is always a risk of it not disengaging at the right moment.
The present invention thus seeks to mitigate such drawbacks by proposing an emergency device for relighting a windmilling turbojet, which device is continuously operational and simple, fast, and reliable to use.
To this end, the invention provides an emergency device for relighting a windmilling turbojet, the jet comprising a fan driven by a low-pressure turbine via a first shaft and a compressor driven by a high-pressure turbine via a second shaft disposed coaxially around the first shaft, said device comprising a differential interconnecting said first and second shafts while compensating for their different speeds of rotation in normal operation or the turbojet, and a braking system connected to the differential so as to enable it to be slowed down or blocked when the turbojet shuts down, thereby enabling the first shaft to entrain the second shaft so that it reaches a speed that favors relighting of the turbojet.
Thus, the emergency relighting device takes energy from the windmilling fan and transfers it to the high-pressure shaft in order to bring it up to a speed that enables the turbojet to be relighted. The differential comprises a planet-carrying annular gear which turns around the two shafts and which is coupled in rotation to a rotary portion of the braking system. This can be constituted by a brake, e.g. a disk brake or a retarder, and it can be controlled by a device under hydraulic, pneumatic, electrical, electromagnetic, or combined control. It is advantageously controlled by an electronic computer enabling the transfer of energy from the low-pressure shaft to the high-pressure shaft to be adapted as a function of the speed of rotation of the high-pressure shaft.